Abstract : We consider simulations of graph automata. We introduce two local transformations on the neighborhood: splitting and merging. We explain how to use such transformations, and their consequences on the topology of the simulated graph, the speed of the simulation and the memory size of simulating automata in some cases. As an example, we apply these transformations to graph automata embedded on surfaces and we link our results with some simulation results between cellular automata on Cayley graphs.